The present invention relates generally to the field of high speed optical communication and more particularly to high speed optical transmission systems utilizing dense wave division multiplexing (DWDM) and other like transmission schemes.
In optical communications systems or optical networks utilizing dense wave division multiplexing (DWDM) and other optical transrnission schemes, a laser diode is used to convert electrical signals to optical pulses for transmission over fiber optic cable. To a certain extent, most laser diodes have one or more of the following drawbacks when used for optical transmission, even without data modulation being applied: 1) Random optical amplitude fluctuations referred to as relative intensity noise (RIN) amplitude; 2) Random optical phase fluctuations; and 3) Random polarization fluctuations which result because of random polarization phase changes or individual random amplitude changes of polarization states or both.
In addition to the above drawbacks, when a laser diode is directly modulated, i.e., the laser diode current is changed in accordance with the digital data to be transmitted, the optical power output (RIN) and optical frequency change randomly, producing what is termed chirp. As is understood by those skilled in the art, chirp broadens the output spectrum of the laser diode. As a result of chirp, transmission distances are shortened and data rates are reduced. Furthermore, optical power limitations may be imposed to limit significant energy utilized as a result of the additional frequencies which are present because of chirp. As would be understood, chirp will depend on modulation waveform types, such as, square wave (maximum chirp), triangular wave, and sinusoidal wave (minimum chirp).
In addition to the above limitations which are present with the laser diode when used in connection with high speed optical communications, the fiber optic cable as used in the transmission introduces certain impairments into the data due to the non-linearities in the fiberoptic cable itself. As would be understood, the impairments may include dispersion, self phase modulation (SPM), cross phase modulation (XPM), FWM, etc.
The high speed optical communications which takes place within the optical systems and networks utilizing the laser diodes and fiberoptic cable may be transmitted using any one of a number of transmission coding schemes. Unipolar return-to-zero (URZ) and unipolar non-return-to-zero (UNRZ) schemes are commonly used in optical data transmission because of their unipolar characteristics. Since laser power is either zero or a certain positive quantity, only unipolar encoding can be implemented in fiber-optical communication systems. UNRZ is a widely used optical communication technique for laser modulation in optical communications because of its low bandwidth requirement as compared to URZ. URZ offers some advantages when used in fiberoptic systems with optical amplifiers; however, this use is at the cost of higher bandwidth.
High speed optical transmitters in the prior art have adopted one or the other coding scheme depending on a particular application and the amount of resources available, e.g., power, bandwidth, etc. The above discussed drawbacks and limitations associated with laser diodes and the corresponding optical networks are present, however, regardless of which of the coding schemes is utilized.
Accordingly, there is a need in the art for an optical transmitting device which combines advantages with respect to transmission of both the URZ and UNRZ coding schemes, while at the same time eliminating those limitations and impairments found in prior art laser diodes and optical networks.
The present invention is a transmitter device and related system and method for use in high speed optical transmission systems. In one exemplary embodiment of the present invention, light from a carrier source, e.g., a laser diode is output to a frequency discriminator which divides the light output into a higher order frequency component and a lower order frequency component. A first external optical modulator receives the lower order frequency components from the laser diode and modulates a first coded data, for example, URZ data from a URZ coder thereon. A second external optical modulator receives the higher order frequency components from the laser diode and modulates delayed first coded data, for example, delayed URZ data representative of the URZ data from the URZ coder thereon. The URZ data and the delayed URZ data may be combined at the transmitter, at any part of the optical cable or at a receiver. The delay at the receiver in the delayed URZ data is preferably T/2, where T is a pulse period of URZ data. The higher order (URZ) and lower order (URZ) frequencies are uniquely added together at one of the above locations in order to realize a second coded data type, for example, UNRZ data. With dispersive optical fiber cable between the transmitter and receiver the higher order and lower order frequency signals encounter propagation delays and arrive at the receiver separately. By changing the optical delay in one or the other or both arms of the receiver, the dispersion effect is compensated without using costly high dispersion fiber, expensive optical amplifiers or other like methods to control dispersion. An advantage of the present invention is that the overall bandwidth of the transmitted data is the same as UNRZ. Transmission of the URZ along with delayed URZ pulses enable one to take advantage of the benefits that these transmission techniques offer for optical networks with optical amplifiers.